Thermally-sensitive triggering mechanism for selective mechanical energization of annular seal element

ABSTRACT

A seal assembly for use in a valve that maintains a seal during operational excursions of high temperature, such as during a fire. The seal assembly includes a eutectic material that retains an energizing element in place during normal conditions, which in turn keeps a secondary seal spaced axially away from a sealing position. During high temperature excursions, the eutectic material degrades and releases the energizing element. When released, the energizing element energizes a secondary seal that is in an annular space between a valve stem and valve housing.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority to and the benefit of co-pending U.S. Provisional Application Ser. No. 61/727,477, filed Nov. 16, 2012; and co-pending U.S. Provisional Application Ser. No. 61/815,978, filed Apr. 25, 2013, the full disclosures of which are hereby incorporated by reference herein for all purposes.

BACKGROUND

1. Field of Invention

This invention relates in general to an annular seal assembly that remains operable during prolonged high temperature excursions. The invention relates in particular to an annular seal assembly that is operable when exposed to high temperature and is in a valve.

2. Description of Prior Art

Flow valves typically have a body with a cavity intersected by a flow passage. A valve element, which is a gate when the valve is a gate valve, moves in the cavity between a closed position, blocking flow through the flow passage and an open position that allows flow through the passage. A stem is usually included for moving the gate, where the stem typically couples to the gate. In manually operated valves, the rotation of the stem causes displacement of the gate between the open and closed positions. In actuated valves, axial movement of the stem results in axial movement of the gate. The stem typically extends through a stem passage in the gate valve assembly. Stem seals are usually provided to seal between the stem and the stem passage and prevent leakage of pressure from the cavity. Often the seals are constructed from an elastomer or polymeric composition that degrades at a temperature lower than what would degrade most of the other valve components.

SUMMARY OF THE INVENTION

Disclosed herein is an example of a valve assembly for use with a wellhead member. In an embodiment, the valve assembly includes a valve housing having a bore, a valve stem inserted into the bore, and a packing assembly for sealing an annulus between the valve stem and valve housing. In this example the packing assembly includes, primary stem packing in the annulus, a secondary stem packing in the annulus that is selectively compressed into sealing contact with the valve stem, a resilient element in the annulus that selectively expands from a compressed configuration to an expanded configuration to exert a compressive force onto the secondary stem packing, a eutectic element that degrades at a temperature greater than a normal operating temperature of the valve assembly, and a compressive stack comprising the eutectic element and that retains the resilient element in the compressed configuration, and releases the resilient element when the eutectic element is exposed to a temperature at which the eutectic element degrades. The compressive stack can project radially through a sidewall of the valve housing and into contact with an energizing element that is axially disposed between the resilient element and the secondary stem packing. In an example, the eutectic element degrades at a temperature that is less than a temperature at which the primary stem packing degrades. The compressive stack can further include a plug on an outer radial side of the eutectic element and a retention element on an inner radial side of the eutectic element. Optionally further included is an annular cartridge in the axial bore having an annular space in which circumscribes the secondary stem packing and resilient member, wherein a lower terminal end of the annular space forms a ledge, and wherein the resilient member is supported on the ledge. In this example, a seal is formed on an outer periphery of the annular cartridge that sealingly engages an inner surface of the bore. In an embodiment, the compressive stack includes the eutectic element and a retention element that impinges against an energizing element. In this embodiment, the eutectic element and an outer radial portion of the retention element are in a groove formed on an inner surface of the axial bore. The compressive stack may have a retention nut that mounts into a side of the valve housing, the retention nut having a bore in which the eutectic member is disposed, the compressive stack further having a retention element that inserts into an end of the bore in the retention nut, the retention element having an end distal from the retention nut that selectively compresses a retention element against the energizing element. This example can further have a port in an end of the retention nut that intersects with the bore in the retention nut, so that when the eutectic element degrades to a flowable substance, the flowable substance can flow from the bore in the retention nut through the port. The valve assembly may further include an annular packing gland for retaining the packing assembly in the valve housing.

Also disclosed herein is an example of a valve assembly for use with a wellhead member that has a valve housing having a bore, a valve stem inserted into the bore, and a packing assembly for sealing an annulus between the valve stem and valve housing. In this embodiment the packing assembly is made up of a secondary stem packing in the annulus that is selectively compressed into sealing contact with the valve stem, a resilient element in the annulus that selectively expands from a compressed configuration to an expanded configuration to exert a compressive force onto the secondary stem packing, a retention element axially adjacent the resilient element and selectively coupled to the valve stem when the resilient element is in the compressed configuration and slidable with respect to the valve stem when the resilient element is in the expanded configuration, a eutectic element that degrades at a temperature less than a temperature at which the primary stem packing degrades, and a compressive stack comprising the eutectic element and that exerts a compressive force to couple together the retention element and valve stem, and that releases the compressive force when the eutectic element is exposed to a temperature at which the eutectic element degrades. This example embodiment may further include an annular cartridge that circumscribes the valve stem and has an upper portion that is spaced radially outward from the valve stem, so that an annular space is formed between the valve stem and annular cartridge for receiving the secondary stem packing, energizing element, and resilient element, and wherein the primary stem packing is in the annulus and below a lower portion of the annular cartridge. This embodiment may further have a retaining element for axially retaining the annular cartridge within the axial bore, wherein inner and outer radial edges of the retaining element project into grooves formed respectively on outer and inner surfaces of the annular cartridge and axial bore. Optionally further included is a gate on an end of the stem for controlling well fluid.

In another example, disclosed herein is a valve assembly for use with a wellhead member that is made up of a valve housing having an axial bore, a valve stem in the axial bore that defines an annulus in the valve housing, a primary stem packing, and a packing assembly in the annulus. The packing assembly includes a secondary stem packing, a resilient member that selectively expands axially into compressive engagement with the secondary stem packing so that the stem packing radially expands into sealing engagement with radial surfaces in the annulus, and a means for retaining the resilient member in a compressed configuration until the valve housing is exposed to a temperature that exceeds a design temperature of the valve assembly by a designated amount.

BRIEF DESCRIPTION OF DRAWINGS

Some of the features and benefits of the present invention having been stated, others will become apparent as the description proceeds when taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a side sectional view of a valve assembly having a thermally energized secondary seal in accordance with the present invention.

FIG. 2 is a side sectional view of the valve assembly of FIG. 1 with the secondary seal energized and in accordance with the present invention.

FIG. 3 is a side sectional view of an alternate embodiment of the valve assembly of FIG. 1 in accordance with the present invention.

FIG. 4 is a side sectional view of an alternate embodiment of the valve assembly of FIG. 1 in accordance with the present invention.

FIG. 4A is an enlarged side sectional view of a portion of the embodiment of the valve assembly of FIG. 4, and in accordance with the present invention.

FIGS. 5 and 6 are side sectional views of alternate embodiments of the valve assembly of FIG. 1 in accordance with the present invention.

While the invention will be described in connection with the preferred embodiments, it will be understood that it is not intended to limit the invention to that embodiment. On the contrary, it is intended to cover all alternatives, modifications, and equivalents, as may be included within the spirit and scope of the invention as defined by the appended claims.

DETAILED DESCRIPTION OF THE INVENTION

The method and system of the present disclosure will now be described more fully hereinafter with reference to the accompanying drawings in which embodiments are shown. The method and system of the present disclosure may be in many different forms and should not be construed as limited to the illustrated embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey its scope to those skilled in the art. Like numbers refer to like elements throughout.

It is to be further understood that the scope of the present disclosure is not limited to the exact details of construction, operation, exact materials, or embodiments shown and described, as modifications and equivalents will be apparent to one skilled in the art. In the drawings and specification, there have been disclosed illustrative embodiments and, although specific terms are employed, they are used in a generic and descriptive sense only and not for the purpose of limitation.

Illustrated in FIG. 1 is a side sectional view of an example embodiment of a valve bonnet assembly 10. A secondary sealing system is included that becomes energized when the valve bonnet assembly 10 experiences an unexpected high temperature excursion, such as being exposed to a fire. The valve bonnet assembly 10 is shown having a body 11 with attached bonnet 12 and mounted in a wellhead member 13; where body 11 and bonnet 12 can be referred to as a housing, either individually or collectively. Examples of the wellhead member 13 include a wellhead housing, production tree, or any other device associated with controlling well fluids. Bonnet 12 as shown includes a bore 14 formed axially therethrough. The valve bonnet assembly 10 includes a gate 15 for selectively blocking a flow of the well fluids; and which is mounted on a lower end of an elongated valve stem 16 shown inserted within the bore 14. The bore 14 extends radially outward from adjacent the stem 16 to define an annulus 17 between the inner surface of bore 14 and outer surface of stem 16. A packing assembly 18 is depicted in a portion of an annulus defined between valve stem 16 and the bore 14. To accommodate the packing assembly 18, a portion of the bore 14 is profiled radially outward and set back from an outer diameter of stem 16. A change in diameter of the bore 14 defines a transition 20 that forms a ledge 21 at the lower end of the profile. A primary stem packing 22 is set in the profiled portion of the bore 14 adjacent transition 20 and supported on the ledge 21. Example materials for the primary stem packing 22 include thermoplastic materials as well as a chevron stem packing.

An annular primary packing gland 24 (also referred to herein as a cartridge) is shown coaxially set in the annulus 17 and which circumscribes a portion of stem 16. An optional channel 25 is formed into the outer periphery of the primary packing gland 24; where the cross section of channel 25 lies oblique to an axis A_(X) of valve bonnet assembly 10 and defines a metal to metal seal between the primary packing gland 24 and bore 14. Alternatively, other means are available to seal between packing gland 24 and bore 14. An axial end of primary packing gland 24 is shown adjacent the primary stem packing 22 and has an inner radius adjacent the stem 16. At an end of the base distal from the primary stem packing 22 the inner radius of the primary packing gland 24 projects radially outward to define an annular space 26 between the primary packing gland 24 and stem 16. An outer radius of the lower portion of primary packing gland 24 projects outward along an oblique path to define a generally conical outer periphery. The upper portion of primary packing gland 24 has a generally constant outer radius and which transitions inward adjacent the lower portion of primary packing gland 24. As shown, the channel 25 is on the upper portion of primary packing gland 24. The annular space 26 extends to an upper terminal end of the primary packing gland 24 distal from primary stem packing 22. A shoulder 27 is defined at the end of the annular space 26 proximate the primary stem packing 22 on which an annular resilient element 28 is supported. The resilient element 28 illustrated is in a compressed state, and can be a Belleville washer, spring, or other material in which potential energy can be compressively stored. The resilient element 28 circumscribes stem 16 and is shown compressed against shoulder 27 of the packing gland 24.

A ring-like energizing element 30 is disposed in the annular space 26 on a side of resilient element 28 axially distal from the shoulder 27. Energizing element 30 is adjacent radial bores 32 that project radially through a side wall of the primary packing gland 24. The bores 32 register with outer bores 34 formed radially outward from an outer surface of bore 14 and through a sidewall of bonnet 12. Retention elements 36 are set in the bores 32, which have an outer radial portion that projects into outer bore 34. In an example, the retention elements 36 are cylindrically-shaped members having a protrusion on an end facing radially inward, protrusion engages a radially inward channel 37 illustrated formed on an outer surface of the energizing element 30. In an example, the energizing element 30 can be made up of two separate rings, the rings having complimentary obliquely angled outer surfaces and placed in a manner that creates channel 37. Further illustrated in FIG. 1 are eutectic elements 38 set in outer bores 34 and on radially outward ends of energizing elements 30. Optionally, plugs 39 are shown mounted in outer bores 34, radially outward from eutectic elements 38, and for providing a radial inward force against eutectic elements 38. Plugs 39 are removable to enable access to retention elements 36 and eutectic elements 38. In the cases where optional plugs 39 are not used, outer bores 34 do not extend to the exterior surface of bonnet 12. The radial inward forces from plugs 39 transfer through eutectic elements 38 to wedge the retention elements 36 against the energizing element 30 and exert a retaining force on the energizing element 30. In an example, the plugs 39, eutectic elements 38, and retention elements 36 define a compressive stack. The retaining force is sufficient to keep spring or resilient element 28 in a compressed state as long as eutectic element 38 is present to provide a backstop for the retention elements 36.

An annular secondary stem packing 40 is shown on an upper end of the energizing element 30 and on a side opposite from resilient element 28. Secondary stem packing 40 is in the annular space 26 between stem 16 and inner wall of primary gland packing 24. Example materials for the secondary stem packing include graphite, as well as Grafoil®. Moreover, example materials for the retention elements 36 include metal and having a melting point greater than the eutectic element 38. A packing gland 42 is shown inserted into an upper end of bore 14, where the packing gland 42 is a generally annular member with a threaded outer circumference that engages threads on an inner circumference of bore 14. As such, engaging the respective threads on the bore 14 and packing gland 42 couples the packing gland 42 to the bonnet 12 and axially retains the other elements of the packing assembly 18 within the annulus 17. Thus, in an example, the packing assembly 18 includes the packing gland 42, the secondary stem packing 40, the eutectic element 38, retention elements 36, energizing element 30, resilient element 28, cartridge 24, and primary stem packing 22.

As discussed above, the eutectic element 38 is made up of a material that melts or otherwise degrades when exposed to high temperature conditions that are unexpected or outside of design parameters, such as a fire. In an alternate embodiment, the eutectic element 38 could degrade a temperature that is less than an expected operating temperature. In an example, material of the eutectic element 38 degrades at a temperature below that at which the primary seal 22 loses sealing function. Referring now to FIG. 2, illustrated is an example of the eutectic element 38 having sufficiently degraded so the retention elements 36 can move radially outward and into the base of the outer bore 34. In an example, degrading of the eutectic element 38 is due to a reduction in the yield strength of the eutectic element 38 such that it deforms under the compressive forces exerted against it. As such, the protruding ends of the retention elements 36 have moved out of engagement with energizing element 30, thereby freeing the energizing element 30 to move axially along the stem 16. Freeing the energizing element 30 by disengaging the retention elements 36, removes the radial inward reactive force exerted by the energizing element 30 onto the retention element 38 and allows resilient element 28 to axially expand; which results in an axial force being exerted against the secondary stem packing 40. In an example, sufficient compressive force is stored in resilient element 28 so that when allowed to extend, the axial load exerted onto secondary stem packing 40 by the expanding resilient element 28 radially expands secondary stem packing 40 into sealing contact with the valve stem 16 and inner surface of the primary packing gland 24. Thus when radially expanded, the resilient element 28 defines a pressure and fluid barrier in the annular space 26, which in combination with the channel 25 creates a seal between stem 16 and bore 14.

Referring now to FIG. 3, shown in a side sectional view is an alternate embodiment of a valve bonnet assembly 10A. In this example, the primary packing gland 24A does not circumscribe other elements of the packing assembly 18A. Instead, the primary packing gland 24A is set in the profile portion of the bore 14A between a lower surface of the resilient element 28A and upper surface of the primary stem packing 22A. Thus, the resilient element 28A is in the annulus 17A created by bore 14A and stem 16A, and second stem packing 40A is in contact with an inner surface of bore 14A. Moreover, an outer radius of the packing gland 42A projects radially inward to define a downward-facing lip 44A supported on a shoulder 46A. In the example of FIG. 3, shoulder 46A is formed where bore 14A projects radially outward. This is in contrast to the lip 44 of FIG. 1 that rests on an upper surface of the primary packing gland 24. Further illustrated in FIG. 3 is an optional retaining element 48A for axially retaining the primary packing gland 24A at an axial location within bore 14A. In the example of FIG. 3, retaining element 48A projects between registered recesses formed respectively in the packing gland 24A and wall of bore 14A. Operation of the example valve bonnet assembly 10A of FIG. 3 is similar to that of the valve bonnet assembly 10 of FIGS. 1 and 2. That is, thermal degradation of eutectic element 38A allows outward radial movement of retention elements 36A, which releases energizing element 30A and allows it to transfer force from axially expanding resilient element 28A to compress secondary stem packing 40A and form a secondary seal around stem 16A before primary seal 22A loses sealing function due to the high temperature. In an alternative, the retention elements 36A may be cylindrically-shaped elements radially oriented with respect to the axis of bore 14A, or segments of a ring having an internal diameter profiled to mate with the outer diameter of the energizing element 30A. The recess 34A may extend the entire circumference of the bore 14A or may be a plurality of recesses. Retention elements 36A are set in the recess(es) 34A

An alternate embodiment of a valve assembly 10B is shown in a side sectional view in FIG. 4. In this example, the retention element 36B is accessible from outside the bonnet 12B to allow easier assembly of the packing stack, and so that disassembly of the valve assembly 10 is unnecessary to replace, repair, or otherwise access retention element 36B. More specifically, a tapered bore 50B is shown extending laterally through a section of the bonnet 12B adjacent the energizing element 30B. A retention assembly 52B is threadingly engaged with the tapered bore 50B and urges retention element 36B radially inward against the energizing element 30B. Similar to the example of FIG. 1, packing assembly 18B is disposed within axial bore 14B and coaxially circumscribing stem 16B. A radius of bore 14B projects radially outward at a transition 20B to define a shoulder for supporting an annular primary stem packing 22B. A collar-like primary packing gland 24B is shown set on an upper end of primary stem packing 22B and circumscribing stem 16B and in bore 14B. The inner radius of the primary packing gland 24B is proximate the outer radius of stem 16B along a lower portion of primary packing gland 24B. The inner radius of primary packing gland 24B then projects radially outward to define an annular space 26B between stem 16B and upper portion of primary packing gland 24B. The shoulder 27B is formed at a lower end of the annular space 26B and provides a support for an annular resilient element 28B in the annular space 26B Annular energizing element 30B sits on an upper end of resilient element 28B and in annular space 26B.

Referring now to FIG. 4A, an enlarged portion of the valve assembly 10B of FIG. 4 is shown in side sectional view. In the example of FIG. 4A, retention element 36B is secured within a retention nut 54B; where retention nut 54B is shown having an outer radial threaded surface which engages threads provided within the tapered bore 50B. The inner radial portion of tapered bore 50B registers with radial bore 32B to allow retention element 36B to project into engagement with energizing element 30B. Proximate to radial bore 32B tapered bore 50B is profiled axially inward at an oblique angle to define an inner shoulder 56B. Set radially outward from inner shoulder 56B, tapered bore 50B angles oblique to an axis A_(X) of tapered bore 50B to define an outer shoulder 58B. Retention nut 54B has a body whose circumference tapers complimentary to the above described tapers in bore 50B. The outer surface of retention nut 54B is shaped to define an inner profile 60B complimentary to inner shoulder 56B and thus interfaces in close contact with inner shoulder 56B when nut 54B is mounted in bore 50B. The interface between inner profile 60B and inner shoulder 56B defines a sealing interface along their area of contact. Radially outward from inner profile 60B, the circumference of retention nut 54B is further tapered to define an outer profile 62B that is angled oblique to the axis A_(X) of retention nut 54B, but as shown in FIG. 4A, outer profile 62B may be radially set outward from outer shoulder 58B so that an annular space is formed between these areas when nut 54B mounts in bore 50B.

A bore 64B is shown formed axially within a radially inward portion of retention nut 54B and that is shown inserted into bonnet 12B. In one example, a seal 66B is set in a recess shown circumscribing an outer surface of retention element 36B, where seal 66B provides a fluid and pressure barrier in the interface between retention element 36B and bore 64B. Further in the example of FIG. 4A, the eutectic element 38B is set in an end of bore 64B distal from energizing element 30B and with retention element 36B between eutectic element 38B and energizing element 30B. In the illustrated embodiment, the eutectic element 38B and retention element 36B are generally cylindrical members, and where the retention element 36B is more elongate than the eutectic element 38B. A port 68B is formed axially through an end of tension nut 54B distal from packing gland 24B; port 68B is shown intersecting with a bottom of bore 64B. Thus, in one example, when eutectic element 38B changes from a solid to a fluid state due to thermal excursions, the now fluid eutectic element 38B can flow out of bore 64B into port 68B and thereby allow retention element 36B to move radially outward and out of engagement with energizing element 30B. As discussed above with respect to FIG. 1, disengaging retention element 36B with energizing element 30B allows element 28B to axially expand and energize secondary stem packing 40B against a lower end of packing gland 42B (FIG. 4) thereby maintaining a seal within bore 14B and along stem 16B.

Referring now to FIG. 5, a side sectional view of an alternate embodiment of valve assembly 10C is shown that includes an annular packing support element 70C within the packing assembly 18C. Further in this example embodiment, the sequence of elements making up the packing assembly 18C begin with primary stem packing 22C on a lower portion of packing assembly 10C, which rests on shoulder 27C within annular space 26C. Mounted on an upper end of primary stem packing 22C is primary packing gland 24C and on an end of primary stem packing 22C opposite shoulder 27C. Similar to the embodiment of FIG. 3, primary packing gland 24C is a generally annular member whose outer circumference gradually extends radially outward with distance away from primary stem packing 22C. Packing support element 70C is shown on an upper terminal end of primary packing gland 24C Annular retaining element 48C is anchored within side walls of annular space 26C, and has an inner circumference that projects radially inward into annular space 26C, thereby axially interfering with the upper end of primary packing gland 24C. The packing support element 70C is in contact with upper surfaces of both primary packing gland 24C and retaining element 48C. Secondary stem packing 40C is shown on an upper end of packing support element 70C on a side opposite primary packing gland 24C.

Retention assembly 52C, which is similar to retention assembly 52B of FIG. 4, is shown set within tapered bore 50C in a side wall of bonnet 12C and for retaining energizing element 30C. A lower surface of energizing element 30C is in contact with an upper surface of secondary stem packing 40C and an upper surface of energizing element 30C is in contact with a lower surface of resilient element 28C. Resilient element 28C is set in annular space 26C, and axially between energizing element 30C and a lower end of packing gland 42C. Thus, in this example, when high temperature excursions melt the eutectic element 38C thereby moving retention force of the retention assembly 52C against energizing element 30C, axial expansion of resilient element 28C merges energizing element 30C in a downward direction to energize secondary stem packing 40C against packing support element 70C and retaining element 48C.

An alternate embodiment of valve assembly 10D is shown in side sectional view in FIG. 6. In this example, retention assembly 52D, which is similar to retention assembly 52B of FIG. 4, mounts in a side wall of bonnet 12D for selectively retaining energizing element 30D axially in place with respect to bonnet 12D. Here, resilient element 28D is set on an upper end of energizing element 30D and within annular space 26D defined between stem 16D and an inner radius of primary packing gland 24D. Secondary stem packing 40D, also in annular space 26D, is on a lower side of energizing element 30D and opposite from resilient element 28D. Retention assembly 52D engages energizing element 30D above channel 25D. Thus, in this example, an energizing element 30D is selectively released by retention assembly 52D, the constant axial force exerted thereon by resilient element 28 results in an axial energizing force to be exerted against secondary stem packing 40D and provide sealing in the event primary stem packing 22D fails.

Disclosed herein are seals for use in wellhead gate valves that maintain high integrity during normal operation well control, and contain pressure in the event of a fire. Compression loaded high temperature seals have limited ability to withstand the rigorous performance validation requirements of a gate valve that function is best served by the qualified normal operation seals. This invention deploys the high temperature seal only during an emergency scenario thereby providing for an emergency seal which is deployed without any degradation due to normal wear.

The present invention described herein, therefore, is well adapted to carry out the objects and attain the ends and advantages mentioned, as well as others inherent therein. While a presently preferred embodiment of the invention has been given for purposes of disclosure, numerous changes exist in the details of procedures for accomplishing the desired results. Examples exist wherein the seal assembly is used as a stem seal for any type of a valve, such as ball valves, plug valves, globe valves, control valves, and the like. Further optional applications for the seal assembly disclosed herein includes usage with chokes, a piston rod in a blow-out preventer. Other embodiments exist wherein the seal assembly is used in conjunction with turbo-machinery and or reciprocating compressors, where specific applications include annular seals around shafts, and annular seals around piston rods that penetrate pressure boundaries. The use of the assembly described herein has applications to a wide variety of fields and is not limited to the oil and gas industry. These and other similar modifications will readily suggest themselves to those skilled in the art, and are intended to be encompassed within the spirit of the present invention disclosed herein and the scope of the appended claims. 

What is claimed is:
 1. A valve assembly for use with a wellhead member comprising: a valve housing having a bore; a valve stem inserted into the bore; and a packing assembly for sealing an annulus between the valve stem and valve housing comprising, primary stem packing in the annulus; a secondary stem packing in the annulus that is selectively compressed into sealing contact with the valve stem, a resilient element in the annulus that selectively expands from a compressed configuration to an expanded configuration to exert a compressive force onto the secondary stem packing, a eutectic element that degrades at a specified temperature other than an expected operating temperature of the valve assembly, and a compressive stack comprising the eutectic element and that retains the resilient element in the compressed configuration, and releases the resilient element when the eutectic element is exposed to a temperature at which the eutectic element degrades.
 2. The valve assembly of claim 1, wherein the compressive stack projects radially through a sidewall of the valve housing and into contact with an energizing element that is axially disposed between the resilient element and the secondary stem packing.
 3. The valve assembly of claim 1, wherein the eutectic element degrades at a temperature that is less than a temperature at which the primary stem packing degrades
 4. The valve assembly of claim 1, wherein the compressive stack further comprises a plug on an outer radial side of the eutectic element and a retention element on an inner radial side of the eutectic element.
 5. The valve assembly of claim 1, further comprising an annular cartridge in the axial bore having an annular space in which circumscribes the secondary stem packing and resilient member, wherein a lower terminal end of the annular space forms a ledge, and wherein the resilient member is supported on the ledge.
 6. The valve assembly of claim 5, further comprising a metal to metal seal formed on an outer periphery of the annular cartridge that sealingly engages an inner surface of the bore.
 7. The valve assembly of claim 1, wherein the compressive stack comprises the eutectic element and a retention element that impinges against an energizing element.
 8. The valve assembly of claim 7, wherein the eutectic element and an outer radial portion of the retention element are in a groove formed on an inner surface of the axial bore.
 9. The valve assembly of claim 1, wherein the compressive stack comprises a retention nut that mounts into a side of the valve housing, the retention nut having a bore in which the eutectic member is disposed, the compressive stack further comprising a retention element that inserts into an end of the bore in the retention nut, the retention element having an end distal from the retention nut that selectively compresses a retention element against the stem.
 10. The valve assembly of claim 9, further comprising a port in an end of the retention nut that intersects with the bore in the retention nut, so that when the eutectic element degrades to a flowable substance, the flowable substance can flow from the bore in the retention nut through the port.
 11. The valve assembly of claim 1, further comprising an annular packing gland for retaining the packing assembly in the valve housing.
 12. A valve assembly for use with a wellhead member comprising: a valve housing having a bore; a valve stem inserted into the bore; and a packing assembly for sealing an annulus between the valve stem and valve housing comprising, a secondary stem packing in the annulus that is selectively compressed into sealing contact with the valve stem, a resilient element in the annulus that selectively expands from a compressed configuration to an expanded configuration to exert a compressive force onto the secondary stem packing, a retention element axially adjacent the resilient element and selectively coupled to the valve stem when the resilient element is in the compressed configuration and slidable with respect to the valve stem when the resilient element is in the expanded configuration, a eutectic element that degrades at a temperature greater than an expected operating temperature of the valve assembly, and a compressive stack comprising the eutectic element and that exerts a compressive force to couple together the retention element and valve stem, and that releases the compressive force when the eutectic element is exposed to a temperature at which the eutectic element degrades.
 13. The valve assembly of claim 12, further comprising an annular cartridge that circumscribes the valve stem, and has an upper portion that is spaced radially outward from the valve stem, so that an annular space is formed between the valve stem and annular cartridge for receiving the secondary stem packing, energizing element, and resilient element, and wherein the primary stem packing is in the annulus and below a lower portion of the annular cartridge.
 14. The valve assembly of claim 13, further comprising a retaining element for axially retaining the annular cartridge within the axial bore, wherein inner and outer radial edges of the retaining element project into grooves formed respectively on outer and inner surfaces of the annular cartridge and axial bore.
 15. The valve assembly of claim 12, further comprising a gate on an end of the stem for controlling well fluid.
 16. The valve assembly of claim 12, wherein the eutectic element degrades at a temperature less than a temperature at which the primary stem packing degrades.
 17. A valve assembly for use with a wellhead member comprising: a valve housing having an axial bore; a valve stem in the axial bore that defines an annulus in the valve housing; a primary stem packing; a packing assembly in the annulus that comprises a secondary stem packing, a resilient member that selectively expands axially into compressive engagement with the secondary stem packing so that the stem packing radially expands into sealing engagement with radial surfaces in the annulus, and a means for retaining the resilient member in a compressed configuration until the valve housing is exposed to a temperature that exceeds a design temperature of the valve assembly by a designated amount. 